The present invention relates to magnetic storage devices and read and write heads for tape drives, and particularly to a method and apparatus for achieving uniform head to tape contact at all tape track positions.
An important requirement in the field of recording on magnetic tape is that the tape is in physical contact with the head for reading and writing. This requirement is different for magnetic recording on a hard disc, for example, where there is a space between the recording medium and the head. In high density recording, proper head-to-tape contact is of extreme importance, since a separation between the head and the tape on the order of magnitude of only a few nanometers creates a significant loss of signal amplitude from the head, due to the so called "distance loss". In order to achieve proper contact between the magnetic head and the tape, various methods have been used.
One attempt at maintaining sufficient head-to-tape contact has been the use of a vacuum, such that a suction at each side of the write or read head keeps the tape in a very well defined and precisely controlled contact with the head. Although this method is efficient, the apparatus is too complex to accommodate easily into smaller form factors, low profile tape drives, in use today. Another method of assuring adequate tape to head contact is to use a pressure pad on a side of the tape opposite to the head and press the tape into contact with the head. The disadvantage of this method is the difficulty of obtaining a uniform contact and surface pressure between the head and the tape due to irregularities in the material of the pressure pad, or the fact that dust particles may collect on the pressure pad and produce a variation of the surface pressure over the area of the pressure pad.
A third method of achieving sufficient tape-to-head contact is to tension the tape during its circulation, while arranging the head to protrude into the tape path so that a force proportional to the tape tension retains the tape in contact with the magnetic head. This method is relatively simple to implement and has been a very efficient way to provide proper head to tape contact. The tape may be tensioned in different ways such as by mechanical friction or by electrical control of the motors that are connected to the tape hubs.
Another known method of achieving a precise control of the tape tension, such as in a magnetic recording device with the tape hubs directly driven by electric motors, such as is a professional open-real tape recorder, is to position a spring-loaded bar or pin made of a completely non-magnetic material into the tape path. If such a tension control device is designed properly, the force that the pin exerts on the tape is by and large independent of the position of the pin, and the pin is oriented such that the centered line coincides completely with a line in the plane formed of the tape surface, that is, perpendicular to the direction of movement to the magnetic tape when the tape is not in contact with the tensioning device. Thus, this tension control device will compensate random variations in the tape tension below the resonance frequency, that is determined by the moveable mass of the mechanical tensioning device and the spring constant. Such variations in the tape tension may for example be produced by random variations of the friction coefficient as the tape moves through the tape guiding mechanism.
A problem relating to the method of tensioning of tape in order to achieve proper head to tape contact, is that the tape tension is sometimes not distributed equally over the transverse cross section of the tape. This unequal distribution of tape tension is due to the fact the thickness and other properties of the tape base film vary along the tape but also because the tape is slitted with limited accuracy. Tape manufacturers continuously work to increase the quality of the base film and precision of the slitting process in the production of todays high performance computer tapes, however variations in slitting of the tape causes the center line of the tape to deviate from a straight line when it is laid down freely on a surface, and consequently a non-uniform tension along the cross section of the tape is produced when the tape is tensioned and forced into a straight guiding mechanism.
A non-uniform tape tension, or a varying "transversal tension-profile" of the tape may result in deficient head-to-tape contact with resulting loss in signal amplitude. Due to this factor, the tape tension must be increased to a level that generally is significantly higher than the level that would be required for a perfectly slitted tape with a completely uniform base film. However, increased tape tension is not desirable because of increased wear of the tape surface as well as the tape edges, increased wear of the magnetic head, higher generation of heat in the tape guiding mechanism, and increased power dissipation of the electrical motors or mechanical tape tensioning devices. The mechanical stress and wear of the tape edges is caused by the forces that act on the tape due to the mentioned imperfections of the slitting process and variations of the properties of the base film, and these forces increase with increasing tape tension. Nevertheless, the current requirement for even higher recording densities requires a precisely controlled level of tape tension along the complete cross section of the tape, while reducing mechanical wear of the magnetic head and tape is reduced to a minimum.